This application claims the priority of Korean Patent Application No. 2003-18502, filed on Mar. 25, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a bipolar plate, and more particularly, to a fuel cell that has a lower internal pressure and can produce a high power.
2. Description of the Related Art
Fuel cells are electrochemical systems in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. A fuel cell consists of three elements, i.e., an anode where reaction takes place, a cathode where a reduction reaction takes place, and an electrolyte disposed between the anode and the cathode to carry cations. In the anode, a fuel, such as hydrogen or methanol, loses electrons and transform into cations. In the cathode, cations accept the electrons supplied from the anode.
FIG. 1A is a plan view of a conventional anode bipolar plate for a fuel cell, in which a fuel flow field is formed. FIG. 1B is a plan view of a conventional cathode bipolar plate having an air flow field.
Referring to FIGS. 1A and 1B, in a conventional fuel cell, a fuel flow field 13 and an air flow field 23 are 15 times or more longer than one side of first and second bipolar plates 11 and 21, which are square, respectively. A fuel inlet 12 and an air inlet 22 are diagonally opposite to a fuel outlet 14 and an air outlet 24, respectively. The fuel flow field 13 and the air flow field 23 are bent at right angle forming a serpentine pattern. Reference numeral 26 denotes a cooling fin.
The fuel flow field 13 and the air flow field 23 may be formed of a metallic network. In most cases, a graphite block with an engraved flow field, the graphite block being electrically conductive and having a predetermined thickness to tighten gas, is used for the fuel flow field 13 and the air flow field 23. The fuel flow field 13 and the air flow field 23 require complicated designs to evenly and continuously supply a fuel or an oxidant from the outermost cell to the innermost cell of the cell stack. In addition, since a plurality of bipolar plates having such fuel and air flow fields are stacked into a fuel cell stack, it is difficult to hermetically seal the fuel cell stack. The conventional fuel cell stack has greater internal pressure variations, and the concentration of reactants is not uniform over the same reaction plate.
FIG. 2 is a graph of voltage distribution in a conventional fuel cell stack comprising a plurality of cells. Cells 9 and 12 show a great drop in voltage at 0.05V and 0.08V, respectively. As is apparent from FIG. 2, due to different voltages of the individual cells, the conventional fuel cell stack shows inconsistent voltage levels.
FIG. 3 is a graph illustrating changes in voltage (E) and power versus current in a conventional fuel cell stack. Referring to FIG. 3, the conventional fuel cell stack has a voltage of 5V at a current of 6A and a maximum power of about 30 W at that voltage. However, a lower voltage leads to a greater drop in power.
To solve these problems, U.S. Pat. No. 6,132,895 discloses a fuel cell with double sided distribution plates having liquid fuel channels and oxidant channels that act as heat exchangers. U.S. Pat. No. 6,127,058 discloses a fuel cell using a plastic frame with a current collector and including an anode and a cathode acting as a gas diffusion layer and a heat management layer, respectively. U.S. Pat. No. 5,527,363 discloses a fuel cell including compressible electroconductive plates with an intermediate plate therebetween, in which one of the compressible plates is embossed to effectively pressurize fluid.
Such conventional fuel cells can be manufactured at low costs through various structural modifications for multiple functions. However, due to a rise in internal pressure, reactions in the fuel cell retard and fuel and oxidant cannot be blocked from being mixed together and leaking. In addition, at a corner of the fuel cell where fluid changes direction, the fluid stays due to the formation of vortices, obstructing smooth fuel flow.